Two generations of klystrons reunited

As the newest accelerator on the block, Linac4 is a hotbed of fresh technology and innovation. But among its many new elements you’ll find some familiar pieces, including eleven klystrons from CERN’s former flagship, LEP.

View of the Linac4 hall. The LEP klystrons (front and centre of the image) are surrounded by grey lead shielding. (Image: Suitbert Ramberger.)

The Linac4 accelerator is powered by both new, state-of-the-art klystrons and former LEP klystrons. In fact, the first Drift Tube Linac (DTL) module is powered completely by these LEP klystrons. The last of the DTL modules has only just been installed in the Linac4 tunnel – a milestone that will soon take the accelerator up to 50 MeV, allowing it to act as a back-up machine for Linac2 for a few years before the complete handover to the CERN accelerator chain.

It’s been a long journey to this point. Linac4 was first conceived in the early 2000s, and its design overlapped with the end of the LEP era. “While we were dismantling LEP, we kept aside 44 klystrons that we knew could be reincorporated into other projects – Linac4 being the chief among them,” says Olivier Brunner, who led the team responsible for the LEP high-power RF system. “As Linac4 was still on the drawing board, its klystron frequency could be chosen to match that of LEP klystrons.”

For the decade-long wait, the LEP klystrons were kept under vacuum and monitored closely. During this time, they were adapted to accommodate Linac4’s pulsed RF operation: “LEP klystrons were designed for a continuous wave machine,” says Brunner, “and so we had to modify them for pulsed operation. They then passed high-voltage tests and were revalidated, ready for their installation in Linac4.”

Suitbert Ramberger, project engineer for the Linac4 DTL, with the third and final DTL module. (Image: Stephan Russenschuck.)

Just like old light bulbs, klystrons are consumables that, eventually, have to be replaced. The team has ten additional LEP klystrons available as back-ups, which will be validated once the Linac4 installation has been completed. However, once all the LEP klystrons reach the end of their lifetime, they will be replaced with new klystrons - one new for every two old.

As for the other remaining LEP klystrons? Most have found new homes around the world, from China to Sweden to France. While the LEP era may have passed, the legacy of the machine lives on!